Studies of the effect of reproductive age on the incidence of mammary cancer induced in the rat by 7,12-dimethylbenz(a)anthracene (DMBA) will be initiated. Furthermore, a method developed in our laboratory for the enzymatic separation of mammary parenchymal tissue from the adipose tissue of the mammary gland fat pad of the rat will be used in these studies to provide mammary gland parenchymal and tumor cells. Both the in vivo and in vitro uptake and retention of DMBA by the mammary parenchymal cells of animals will be determined as an indication of an alteration in carcinogen susceptibility of parenchymal cells of animals undergoing their first pregnancy at different ages. Additional in vivo and in vitro studies to determine the hormonal responsiveness of mammary parenchymal and tumor cells will be undertaken. Moreover, studies to determine the effect of pregnancy and estrogens on mammary cancer in animals exposed to DMBA transplacentally are also proposed.